The present invention relates to a dot-matrix printer of a serial type with a character generator for storing dot character patterns input by an external device, and for generating the patterns of dot characters to be printed.
A known dot-matrix printer of a serial type comprises a character generator, a print head-driving circuit and a print head. The printer receives a character code from the external device (usually a host computer). The character generator generates the dot character pattern corresponding to the code. The character patterns are supplied to the print head-driving circuit. This print head driving circuit drives the printing pins of the print head, thereby printing a dot character as shown in FIG. 1.
Recently it has been increasingly demanded that a dot-matrix printer print dot characters of better quality. To meet this demand, a new dot-matrix printer has been invented which can print a character in high density as shown in FIG. 2. The dots forming this character are arranged at half the pitch of those forming the character of FIG. 1, in the direction F in which the print head moves to print dot characters.
The printing speed of the dot-matrix printer is usually determined by the maximum response frequency of the print head, i.e., the frequency at which the head moves in direction F. More specifically, the speed is given as f/N [cps], where f is the maximum response frequency of the head and N is the number of dots of each row of the dot matrix. Since the dots are arranged in direction F at half the pitch of those forming the character shown in FIG. 1, the printing speed of the printer is half that of the ordinary dot-matrix printer.
When a dot-matrix printer prints dots at a far smaller pitch than those forming the character of FIG. 1, as shown in FIG. 3A, its printing speed is given as f/N [cps], where f is the maximum response frequency of the printer. To raise the printing speed, N may be decreased. Hence, when the printer prints dots at half the pitch as shown in FIG. 3B, the speed rises to 2f/N [cps]. When it prints dots at one-third the pitch as illustrated in FIG. 3C, it rises to 3f/N [cps]. Yet the printed characters have better quality since the dots arranged at the pitches (FIGS. 3B and 3C) are far smaller than the dots forming the character of FIG. 1.
The character generator and provided in a dot-matrix printer is a ROM (read-only memory) in most cases. The ROM stores the dot patterns corresponding to character codes. To print a special character whose dot pattern is not stored in the ROM, the ROM must be replaced by a ROM which stores the pattern of the special character. It is troublesome to replace one ROM with another, however. To avoid his trouble, a dot-matrix printer has been invented which can register the dot patterns of special characters through an external device. This printer is equipped with a character register/generator comprising a RAM (random-access memory) which can store the dot pattern of any character other than those stored in the ROM, which has been input through the external device. The printer can thus print the dot character of any pattern input through the external device.
The dot-matrix printer described in the preceding paragraph operates in the following manner. First, the external device supplies the printer with a pattern registration command. The printer therefore becomes ready to register any dot character pattern. Then, a character code, and data showing character pitch L1 (FIG. 4) and character width L2, both shown in FIG. 4, are supplied from the external device to the printer. The character width L2 determines the number of bytes representing the dot character pattern to be registered, and the character code determines the address of the RAM in which this pattern is to be registered. The dot character pattern is supplied to the RAM and is thus registered therein.
To print a dot character whose pattern has been input through the external device at a high speed the pattern must be registered in the form shown in FIG. 3B. However, the more complex the dot pattern is, the more often it is registered in an erroneous form. When the pattern is erroneously registered such that some of its dots are arranged as densely as shown in FIG. 3A, the pins of the print head may be broken during high-speed printing.